1. Field of the Invention
The present invention relates to an image processing apparatus that collectively processes image data with regard to a plurality of pages by halftone processing, a corresponding method of processing images, a printing apparatus to which the technique of image processing is applied, and a recording medium in which a program for executing the technique of image processing is recorded.
2. Description of the Related Art
The ink jet printer, which creates dots using a plurality of different color inks ejected from a plurality of nozzles provided on a print head and thereby records an image, has been proposed as an output device of a computer and widely used to print an image processed by the computer in a multi-color, multi-tone manner. This printer enables an image to be printed both on standard-sized printing medium, such as a sheet of cut paper, and a large-sized printing medium, such as a roll of machine glazed paper. The latter case is hereinafter referred to as continuous printing. A large image may be printed on a printing medium of several ten meters, for example, a banner.
There is generally a restriction in quantity of image data processed at once by an application program. In the case of continuous printing, the application program accordingly supplies print data divided in a plurality of pages. In the case of standard printing, a certain margin is present between adjoining pages. The process of continuous printing removes the margin between the adjoining pages and thereby enables an integrated image to be printed while receiving image data divided into a plurality of pages.
The ink jet printer generally enables only binary expression, that is, either a dot on state or a dot off state, with regard to each pixel. The tone of image data is accordingly expressed by a dispersion of dots. The halftone processing sets the dot on-off state of each pixel based on the tone values of the image data, in order to attain such expression. A multi-value printer, which has been proposed recently, enables expression of tone values by ternary or greater notation with regard to each pixel by creating dots of different ink weights or by creating dots with inks of different densities. In the multi-value printer, the tone value expressible in each pixel is lower than the tone value of the image data, so that the halftone processing is carried out to express the respective tone values by the dispersion of dots. An error diffusion method and a dither method are known techniques applied for the halftone processing.
In the case of continuous printing, image data divided into a plurality of pages are supplied to the printer driver as discussed above. While the application program supplies image data of each page with a start-of-page code and an end-of-page code, the printer driver can not grasp the total number of pages until a code representing the end of image data is input. In order to carry out the image processing and printing procedure smoothly in parallel with the input of image data under such circumstances, the continuous printing process carries out the image processing and printing procedure independently for each page.
The continuous printing process divides a set of image data, which has been generated collectively and integrally, into a plurality of pages without taking into account a linkage of a resulting image. The independent image processing and printing procedure for each page results in undesirably lowering the picture quality on the boundary between adjoining pages. For example, the dispersion of dots unnaturally changes on the boundary of pages, and causes an unnatural variation in tone value. In another example, the unnatural change of the dispersion of dots causes a quasi-outline.